1. Field of the Invention
The present invention relates to a method for producing a liquid developer, a liquid developer, and an image forming apparatus for use with the liquid developer.
2. Description of the Prior Art
As a developer used for developing an electrostatic latent image formed on a latent image carrier, there are known two types. One type of such a developer is known as a dry toner which is formed of a material containing a coloring agent such as a pigment or the like and a binder resin, and such a dry toner is used in a dry condition thereof. The other type of such a developer is known as a liquid developer which is obtained by dispersing toner particles into a carrier liquid having electric insulation properties.
A dry toner is generally produced using a dry grinding method in which a coloring agent and a material containing a binder resin are ground in a dry condition. In the developing method using such a dry toner, since a solid state toner is used, there is an advantage in handleability thereof. On the other hand, however, this method involves problems in that contamination is likely to be caused by dispersal of toner powder and toner particles are likely to be massed together in a cartridge. Further, in such a dry toner, since aggregation of toner particles is likely to occur during preservation and the like, it is difficult to obtain toner particles each having a sufficiently small diameter. This means that it is difficult to form a toner image having high resolution. Furthermore, there is also a problem in that when the size of the toner particle is made to be relatively small, the problems resulted from the powder form of the dry toner described above become more serious.
On the other hand, in the developing method using a liquid developer, since aggregation of toner particles in the liquid developer during preservation is unlikely to occur comPared to the dry toner due to the use of an insulation liquid, it is possible to use very fine toner particles. As a result, compared to the method using the dry toner, the method using the liquid developer has such advantages as good reproductivity of an image composed of thin lines, good tone reproductivity as well as good reproductivity of colors. Further, the method using the liquid developer is also superior as a method for forming an image at high speed.
As a method for producing such a liquid developer, there in known a wet grinding method in which a material containing a coloring agent and a resin is ground in an electric insulation liquid to thereby produce a liquid developer (one example of such a method is disclosed in JP-A No. 8-36277).
Although a recent trend of a higher-resolution image requires further finer toner particles, it is difficult to grind toner particles into sufficiently small pieces using such a conventional liquid developer producing method. Further, in the conventional liquid developer producing method, in order to produce toner particles having a sufficiently small size, a considerably large amount of grinding energy is required for a prolonged period of time, thus resulting in significantly low productivity of a liquid developer. In addition, according to the above-mentioned conventional method, particle size distribution of toner particles is likely to become large, namely toner particles are likely to have large variations in their particle sizes. As a result, relatively large variations are likely to be caused in the properties of the toner particles such as chargeable characteristics thereof. Instead of the wet grinding method, the dry grinding method described above may be used for producing a liquid developer. In this case, however, it is extremely difficult to obtain fine particles suitable for toner particles for a liquid developer. That is, it is difficult to obtain toner particles having a relatively small size since aggregation of toner particles is likely to occur.
Further, in the conventional wet grinding method, it is difficult to obtain a liquid developer in which dispersibility of toner particles is sufficiently high. In a case where dispersibility of toner particles is insufficient, there is a problem in that toner particles settle down when being left for a long period of time, whereby causing aggregation of toner particles. Additionally, if once aggregation of toner particles occurs, it is difficult to re-disperse the aggregated toner particles by agitation. Therefore, there is a problem in that toner particles cannot be supplied uniformly when forming an image.